Process for removing sodium sulfate from aqueous buffer solutions

ABSTRACT

Process for removing sodium sulphate from an aqueous citrate, adipate or phosphate buffer solution used for absorption of SO 2  from exhaust gases, by evaporating an aqueous buffer solution at atmospheric pressure at a temperature near its boiling point, thereafter separating the precipitated sodium sulphate and optionally subjecting it to further treatment to recover accompanying buffer salt. The further treatment may consist in a treatment of the precipitated sodium sulphate together with accompanying buffer salt with water at elevated temperature. The solution which is then obtained is mixed with the rest of the buffer solution.

This invention relates to a process for purifying aqueous buffer solutions used for absorption of sulphur dioxide (SO₂) from exhaust gases. "Purifying" includes in this case also the recovery of buffers from a drawn off liquid flow. Such a drawing off is in many cases necessary in order to prevent impurities which accumulate in the process liquor, from building up to too high concentrations. The most important impurity will be sodium sulphate (Na₂ SO₄). but other components may also be present.

Impure solutions of the above type are for instance obtained when phosphate, citrate or adipate buffers are used for SO₂ recovery from exhaust gases in coal heated boiler plants. In such uses SO₂ is absorbed in the buffer solution to form bisulphite ions (HSO₃ ⁻). In such process acidic hydronium ions (H₃ O⁺) are liberated which are then taken up by buffer ions to form more of the acidic buffer components. The following reactions are utilized in the absorption plant when for instance phosphate ions represent the buffer system:

    SO.sub.2 (g)=SO.sub.2(l)                                   ( 1)

    SO.sub.2 (l)+2H.sub.2 O.sub.(l) =H.sub.3 O.sup.+.sub.(l) +HSO.sub.3.sup.-.sub.(l)                                  ( 2)

    H.sub.3 O.sup.+.sub.(l) +HPO.sub.4.sup.2-.sub.(l) =H.sub.2 PO.sub.4.sup.-.sub.(l) +H.sub.2 O.sub.(l)                 ( 3)

    SO.sub.2(g) +H.sub.2 O.sub.(l) +HPO.sub.4.sup.2-.sub.(l) =HSO.sub.3.sup.-.sub.(l) +H.sub.2 PO.sub.4.sup.-.sub.(l)

From the absorption tower the solution is conveyed to the regeneration plant, in which the liquid in this case is evaporated according to Norwegian Patent Application 85.3622. In the process SO₂ is driven off together with vaporized water by reversing the reactions (1)-(3), and a concentration and precipitation of Na₂ HPO₄ takes place and possibly also of the oxidation product Na2SO₄, as the buffers are usually employed in the form of Na salts:

    NaHSO.sub.3(l) +NaH.sub.2 PO.sub.4(l) +nH.sub.2 O.sub.(l) =SO.sub.2(g) +(n+1)H.sub.2 O.sub.(g) +Na.sub.2 HPO.sub.4(s)

(n=a high number).

If citrate or adipate buffers are used instead of phosphate buffers, the regeneration does not take place by evaporation but by regular stripping with open steam in counter current towers. Then the regeneration takes place without any essential change in the liquid volume, by reversing the reactions (1)-(3), in which citrate ions and citric acid (Ci³⁻, HCi₂₋, H₂ Ci⁻, H₃ Ci) or adipate ions and adipic acid (Ad²⁻, HAd⁻, H₂ Ad) are included in the reactions (3) instead of the phosphate ions (HPO₄ ²⁻, H₂ PO₄ ⁻).

For all said three processes the oxidation loss of SO₂ is very small. Nevertheless, some increase of Na₂ SO₄ in the liquid must be expected, also because the gas from which SO₂ is removed, often contains small concentrations of SO₃ which are taken up by the buffer solution and which must be neutralized with a Na base (NaOH or Na₂ CO₃) An obvious possibility to maintain the Na₂ SO₄ concentration at an acceptable low level, would be to draw off a small flow of process liquid and replace this by fresh buffer solution. For a 500 MW power plant with 0.25 vol % SO₂ in the exhaust gas and a sulphate formation corresponding to 0.25% of the amount of SO₂, the amount of sulphate will amount to approx. 0.5 kmol/h. If it is desired to keep the sulphate concentration as low as 0.5 kmol/m³ only by removing process liquid, 1 m³ liquid per hour must be drawn off. With typically 3 moles per liter of Na₂ HPO₄ in the buffer at a price of approx. US $ 1.2 per kilogram, the loss of Na₂ HPO₄ alone will represent US $ 511 per hour. Even with this low estimate for the sulphate formation, such a drawing off of process liquid to remove Na₂ SO₄ from the system will lead to unacceptably high losses of Na₂ HPO₄. In addition it may represent considerable environmental problems. This illustrates that there exists a need for being able to remove Na₂ SO₄ selectively from the process liquid, i.e. without losing essential amounts of valuable buffer.

The buffer systems discussed herein are all salts of polyvalent acids. The SO₂ uptake in the absorption tower displaces the buffer systems towards the more acidic buffer components. When reference is made to a selective removal of Na₂ SO₄, this means that no buffer components must be lost in essential amounts together with the Na₂ SO₄ removed. This is a problem to which it has been very difficult to find a satisfactory solution so far, as demonstrated for two cases:

Case 1

In a known process for SO₂ recovery from exhaust gases, SO₂ is absorbed in an aqueous Na₂ SO₃ solution to form NaHSO₃ :

    SO.sub.2(g) =SO.sub.2(l)

    SO.sub.2(l) =Na.sub.2 SO.sub.3(l) +H.sub.2 O.sub.(l) =2 NaHSO.sub.3(l)

The solution is regenerated by evaporation, whereby SO₂ is separated and solid Na₂ SO₃ crystallizes:

    2 NaHSO.sub.3(l) +n H.sub.2 O.sub.(l) =SO.sub.2(g) +(n+1)H.sub.2 O.sub.(g) +Na.sub.2 SO.sub.3(s)

(n=high number).

After dissolution of Na₂ SO₃(s) the buffer is ready for new absorption. For this "sulphite process" relatively high SO₂ losses by oxidation are reported. Na-base is added to neutralize the sulphuric acid formed, and in one modification Na₂ SO₄ is removed from the absorption solution by cooling crystallization at about 0° C. The salt which is then taken out, is reported to contain a high concentration of valuable buffer salts (1). which are thereby lost. In a modification extra SO₂ is added to convert all Na₂ SO₃ to NaHSO₃ in order to improve the selectivity in the Na₂ SO₄ precipitation (2). In another modification Na₂ SO₄ is removed by evaporation crystallization at high temperature. It is reported that this modification produces crystals having a lower Na₂ SO₄ content than the cooling crystallization, which means that it is less selective (3).

Particularly in the evaporation method, solutions having very high salt concentrations will be obtained, and it is then not possible on the basis of solubility data for the individual salts to say, with a reasonable degree of certainty, which selectivity can be obtained. The same uncertainty must also be expected in connection with evaporation of solutions of other salt mixtures.

Case 2

For the above mentioned citrate process, based on SO₂ absorption followed by regular steam stripping, there has been developed a multistep method for removing Na₂ SO₄ from a drawn off flow of process liquid, whereby a very high selectivity is attained (Norwegian Patent 133.546):

Step 1: Removal of Na-citrate by evaporation crystallization.

Step 2: Removal of Na₂ SO₄ by cooling crystallization.

Step 3: Removal of remaining Na-citrate by evaporation crystallization.

It has been argued against this method that it requires extensive process control, and that sodium citrate must be precipitated by evaporation under vacuum (4). Similar problems and objections may also be expected when the method is used for purifying adipate and phosphate buffers instead of citrate buffers, and there is accordingly a need for a simpler method for removing Na₂ SO₄ selectively from spent buffer solutions.

Relevant buffer concentrations are 0.25-3.0M for citrate and adipate buffers, and 2.0-5.0M for phosphate buffers. It is normally desirable to keep the Na₂ SO₄ concentration at no greater than 2.0M, particularly in the range 0.1-2.0M.

On the basis of a comprehensive test program we have now found an unexpectedly simple method for selective removal of Na₂ SO₄. This method may be adapted to the phosphate, citrate and adipate processes described above. The method is based on our observations that the solubility of Na₂ SO₄ in solutions simultaneously saturated with Na₂ SO₄ and one of the buffer salts in question, Na₂ HPO₄, Na₂ HCi or NaHAd, is low at high temperatures, while at the same time the solubility is high for the buffer salts. According to the invention there is provided a process for removing sodium sulphate from aqueous citrate, adipate or phosphate buffer solutions used for absorption of SO₂ from exhaust gases, without essential loss of SO₂ and valuable buffer components. The process is characterized in that the aqueous buffer solution is evaporated at a temperature close to its normal boiling point to precipitate sodium sulphate, and in that the precipitated sodium sulphate is separated from its mother liquor and subjected to a possible further treatment to separate and recover accompanying buffer salts. Evaporated SO₂ and the solution from which the Na₂ SO₄ crystals have been separated, are returned to the main process. Buffer solutions which are subjected to treatment, may herein be buffer solutions coming from the absorption unit in which they have been charged with SO₂, or already regenerated buffer solutions which have been made ready for new absorption. In particular for phosphate buffers the evaporation to remove Na₂ SO₄ may be combined with the previously mentioned evaporation in the main process, in such a way that only an adjusted part of the already evaporated concentrate is taken out for sulphate removal.

The process is demonstrated by the following three examples:

EXAMPLE 1 Citrate buffer

Procedure:

A controlled and known flow of buffer solution is drawn off from the liquid circulation cycle, for instance after the stripping tower. The liquid is evaporated at elevated temperature, i.e. substantially higher than in the first step of the above process for purifying citrate buffers, suitably at 100° C. and atmospheric pressure. During this process Na₂ SO₄ crystallizes selectively. The evaporation is terminated before the Na-citrates have reached their saturation concentration. The evaporation may either be carried out as a continuous or as a discontinuous process. Crystallized Na₂ SO₄ is separated from the mother liquor which is returned to the main process, while the escaping steam, which will contain some SO₂, may for instance be combined with the SO₂ -containing exhaust steam from the stripping tower, so that SO₂ values are not lost. Precipitated Na₂ SO₄ may thereafter be subjected to a new process step in which accompanying buffer components are dissolved in water at elevated temperature and conveyed back to the main process before the final removal of Na₂ SO₄.

The method is demonstrated in the following by evaporation of a Na₂ HCi-rich buffer, but may also be used on buffer solutions rich in NaH₂ Ci and H₃ Ci. The latter buffers may be useful in the citrate process for purification of particularly SO₂ -rich gases.

    ______________________________________                                         Data illustrating the usefulness                                               ______________________________________                                         Volume of buffer solution drawn off:                                                               1000 ml                                                    Composition:        1.00M Na.sub.2 CHi +                                                           0.5M Na.sub.2 SO.sub.4                                     Density of buffer solution:                                                                        1.19 g/ml                                                  Calculated starting composition:                                               Na.sub.2 HCi:       19.8% by weight                                            Na.sub.2 SO.sub.4 :  6.0% by weight                                            ______________________________________                                    

This solution was evaporated at atmospheric pressure. The evaporation was carried out in a glass flask equipped with a water cooled condenser and a flask for collection of the condensate.

Evaporated amount of water was measured to: 768.0 g

Samples were then taken of crystal-free mother liquor for analysis. Weight and volume of the samples were determined before the samples were analyzed with respect to Na, P and S. The following results were obtained for samples taken at 100° C.:

    ______________________________________                                                   S     Na      C       Density (g/ml)                                 ______________________________________                                         Sample 1    0.46    12.98   21.12 1.49                                         (g/100 g sample)                                                               Sample 2    0.68    11.86   18.91 1.44                                         (g/100 g sample)                                                               ______________________________________                                    

From the S- and C-analyses the salt concentrations are calculated to be:

    ______________________________________                                                        Na.sub.2 SO.sub.4                                                                     Na.sub.2 CHi                                             ______________________________________                                         Sample 1 (g/100 g)                                                                              2.04     69.23                                                Sample 2 (g/100 g)                                                                              3.01     61.97                                                ______________________________________                                    

It is calculated that the salt concentrations found in this way correspond to an amount of Na in the samples of 14.2 and 12.3 g/100 g sample, respectively. The Na-analyses are in conformity with these values within reasonable limits of error.

The Na-analyses are considered to be the least accurate of the analyses made.

Mean value for the two samples give for the mother liquor:

65.5% by weight Na₂ HCi

2.5% by weight Na₂ SO₄

ρ=1.47 g/ml.

In comparison the solubility of Na₂ SO₄ in pure water of 100° C. is given as 29.7 g/100 g solution (5), i.e. about 12 times higher than that found in the mother liquor.

Remaining H₂ O in mother liquor:

    (1190-(1.00·236.1+0.50·142)-768.0=114.9 g

From this the following is calculated: ##EQU1##

Based on the above the precipitates as percentages are calculated: ##EQU2##

In the above example it is assumed that Na₂ SO₄ is precipitated by evaporation of the buffer solution taken from the flow after the SO₂ stripping. It is also possible to precipitate sulphate by evaporation of a SO₂ -rich solution drawn off from the process liquor which is conveyed to the stripping tower. However, this may entail a somewhat greater risk of corrosion.

EXAMPLE 2 Adipate buffers

Procedure:

Process liquor for Na₂ SO₄ removal may in this case suitably be drawn off from the flows of process liquor immediately before the absorption column, i.e. after SO₂ has been removed by stripping and after free adipic acid (H₂ Ad) has been removed by cooling crystallization from the return liquid from the stripping tower. (The precipitated adipic acid is returned to the process liquid before stripping to facilitate the removal of SO₂, see Norwegian printed specification 155684). Further processing is conducted according to the description from the citrate process.

    ______________________________________                                         Data illustrating the usefulness                                               ______________________________________                                         Volume of buffer solution drawn off:                                                                  1000 ml                                                 Composition:           1.00M NaHAd +                                                                  0.50 M Na.sub.2 SO.sub.4                                Density of buffer solution:                                                                           1.11 g/ml                                               Calculated composition:                                                                               6.4% by weight                                                                 of Na.sub.2 SO.sub.4.                                   ______________________________________                                    

This solution was evaporated at atmospheric pressure.

Amount of water evaporated off: 790.4 g.

The mother liquor was then analysed as described for the citrate buffer, with the following result:

0.13 g S/100 g of sample

7.45 g Na/100 g of sample

28.64 g C/100 g of sample

density ρ=1.21 g/ml.

On the basis of the S- and C-analyses the following salt concentrations are found:

66.8% by weight of NaHAd

0.58% by weight of Na₂ SO₄

These salt concentrations correspond to a total Na-concentration of 9.33 g/100 g sample. This differs from the analysis by about 25%, which is more than expected on the basis of the assumed uncertainty of the analyses. (As previously mentioned the S- and C-analyses are considered more accurate than the Na-analysis).

Remaining H₂ O:

    (1110-(1.00·168.1+0.50·142))-790.4=80.5 g

From this the following is calculated:

    ______________________________________                                         Weight of mother liquor:                                                                      (80.5/(100 - 66.8 - 0.58))                                                     100 = 246.6 g                                                   Na.sub.2 SO.sub.4 in mother liquor:                                                           246.8 · 0.00058 = 1.43 g                               NaHad in mother liquor:                                                                       246.8 · 0.668 = 164.9 g                                Percentage precipitated:                                                       Na.sub.2 SO.sub.4 :                                                                           100 - (1.43/142 · 0.5) 100 = 98.0%                     NaHAd:         100 - (164.9/168.1) 100 = 1.9%.                                 ______________________________________                                    

In the example it is assumed that the sulphate precipitation takes place by evaporating the process liquid after the SO₂ stripping and adipic acid removal. Alternatively sulphate may be precipitated from SO₂ -rich liquid from the absorption tower and before the H₂ Ad-addition. Evaporation of adipic acid rich solution taken from the flow before or after the stripping tower is also a possibility, but there is a risk of coprecipitation of adipic acid which must be avoided.

EXAMPLE 3 Phosphate buffer

Procedure:

The evaporation for regeneration of the process solution in the absorption-stripping-cycle may take place in two steps by allowing the liquid to pass in series or parallel through the two steps, and the exhaust steam from one of the steps serves as a heating medium for the other step (="double effect" evaporation). During the work with the invention it has been found that the precipitate is surprisingly rich in Na₂ SO₄, in particular this applies to the precipitation in the first step where the steps are arranged in series, and it is sufficient to take out a small part of this precipitate to keep the Na₂ SO₄ concentration in the process liquid at a desired level. Co-precipitated amounts of Na₂ HPO₄ and Na₂ S₂ O₅ will be very small and will normally not represent any values of importance. However, the invention also comprises an unexpectedly simple embodiment in which the co-precipitated salts may generally be separated from Na₂ SO₄ so that the buffer salts may be returned practically quantitatively to the main process.

    ______________________________________                                         Data illustrating the usefulness                                               ______________________________________                                         Start solution     2.50M Na.sub.2 HPO.sub.4                                                       0.10M NaH.sub.2 PO.sub.4                                                       0.50M Na.sub.2 SO.sub.4                                                        1.49M SO.sub.2                                                                 V = 2.70 l                                                                     ρ = 1.37 g/cm.sup.3.                                    ______________________________________                                    

The composition corresponds here to a SO₂ charged buffer from the absorption tower.

The solution is evaporated at atmospheric pressure, and precipitated salts are separated from the mother liquor by filtration in a pressure filter at 100° C., after the majority of the mother liquor has been removed by decanting clear liquid. The evaporation of decanted liquid is continued together with filtrate for new precipitation, etc. The composition of the filter cakes are then found on the basis of analysis of samples with respect to Na, P, total S and S^(IV) -content, after dissolution in water.

Results:

    ______________________________________                                         Evaporation steps                                                                         1      2        3        Sum 1-3                                    ______________________________________                                         (a)                                                                            Weight at the start                                                                         3699     2471.3   2186.7                                          of the evaporation (g)                                                         (b)                                                                            Evaporated before                                                                           1131.8   120.7    238.4  1490.9                                   filtration                                                                     H.sub.2 O + SO.sub.2 (g)                                                       (c)                                                                            Filter cake, 50.1     111.1    108.8  270.0                                    total weight (g)                                                               Na.sub.2 SO.sub.4 (g)                                                                       28.4     55.4     41.2   125.0                                    Na.sub.2 HPO.sub.4 (g)                                                                      9.9      25.6     34.1   69.6                                     Na.sub.2 S.sub.2 O.sub.5 * (g)                                                              6.7      15.2     16.2   38.1                                     (d)                                                                            Weight of decanted                                                                          2471.3   2186.7   1807.0                                          liquid and filtrate (g)                                                        (e)                                                                            Not found                                                                      (a) - ((b) + (c) +                                                                          45.8     52.8     32.5                                            (d) (g)                                                                        ______________________________________                                          *Na.sub.2 S.sub.2 O.sub.5 is an anhydride of NaHSO.sub.3 which only exist      in dissolved form.                                                       

Material which has not been recovered represents 1.2-2.1%, and is presumably due to loss of slurry and small amounts of steam and SO₂ during the filtration.

From this the following is calculated:

    ______________________________________                                         Precipitated Na.sub.2 SO.sub.4,                                                                (125/2.7 · 0.5 · 142) 100 = 65%              proportion of initial amount                                                   1) Proportion by weight of                                                                     (125/270) 100 = 46%                                            Na.sub.2 SO.sub.4 in the filter cake:                                          2) Proportion by weight of                                                                     (69.6/270) 100 = 26%                                           Na.sub.2 HPO.sub.4 in the filter cake:                                         3) Proportion by weight of                                                                     (38.1/270) 100 = 14%                                           Na.sub.2 S.sub.2 O.sub.5 in the filter cake:                                   Sum 1) + 2) + 3) = 86%                                                         ______________________________________                                    

The missing 14% is assumed to be primarily water. It is remarked that the filter cakes were not washed. Washing would have contributed to reduce the content of phosphate and disulphite in the cakes, since some mother liquor has been included in the analysis.

With reference to the power plant example discussed initially, the maintenance of 0.50M Na₂ SO₄ in the process liquid will in this case require a salt removal of X kg/h, in which

    X·0.65/142=0.5 kmol Na.sub.2 SO.sub.4 /h.

From this follows: X=109 kg/h.

This removal is accompanied by

    Na.sub.2 HPO.sub.4 : 109·0.26=28 kg/h

    Na.sub.2 S.sub.2 O.sub.5 : 109·0.14=15 kg/h.

The value of the amount of phosphate is estimated on the basis of the price quoted above to 20·1.2=49 US $/h. This amount represents a negligible proportion of the costs connected with the SO₂ process.

It it further remarked that totally 79.7 g SO₂ and 1411.2 g H₂ O have been removed by the evaporation in the example given. In comparison the content of H₂ O in the initial solution is calculated to be:

    2.7 (1370-(2.5·142+0.1·120+0.5·142+1.49·64))=2259 g.

According to this the amount of water evaporated off is:

    (1411.2/2259)100=62 % of the initial amount.

Thus, the amount of water removed by evaporation is almost the same as that which would be suitable to evaporate in the first step in a two step evaporation process.

The example given herein was started with 0.1 mole/1 of the acidic buffer salt NaH₂ PO₄. It has been found that the method is equally suitable for higher concentrations of this salt. As an example, after evaporating off 1385.9 g H₂ O+SO₂ from a buffer with the initital composition 2.5M Na₂ HPO₄ +0.50M NaH₂ PO₄ +0.50M Na₂ SO₄ +1.20M SO₂, 193.5 g of filter cake was filtered off, with a composition 48% Na₂ SO₄, 31% Na₂ HPO₄ and 18% Na₂ S₂ O₅.

Separation of co-precipitated buffer components from Na₂ SO₄

Examples 1-2 above illustrate very good selectivity for the sulphate precipitation from the citrate and adipate buffers respectively, while example 3 indicates a lower selectivity for the phosphate buffer. In particular when phosphate buffers are used, but also with adipate and citrate buffers, for instance when the process control is not as good as illustrated in examples 1 and 2, particularly so that the evaporation goes much further than illustrated above, an embodiment will therefore be desirable which ensures that a negligible proportion of buffer salts are lost together with precipitated Na₂ SO₄. In particular for the phosphate buffer this may be the case when the Na₂ SO₄ removal is started with finally evaporated and concentrated buffer/slurry from the regeneration part of the main process. It is thereby achieved that the removed Na₂ SO₄ has a high purity and thereby becomes simpler to sell commercially, or deposit. The essence of this part of the invention is that the Na₂ SO₄ precipitate from the evaporation step is subjected to a treatment with pure water at elevated temperature for selective solution of co-precipitated buffer salts. The solution which is thereby obtained is mixed with the rest of the buffer solution. The method is demonstrated with the following examples of results in laboratory experiments.

EXAMPLE 4 Citrate buffer

    ______________________________________                                         Initial mixture of solid salts:                                                                    Na.sub.2 HCi                                                                               25 g                                                               Na.sub.2 SO.sub.4                                                                         100 g.                                          ______________________________________                                    

This mixture corresponds to a sulphate precipitation from a too advanced evaporation.

40 g of H₂ O were added to the "precipitate" in a closed glass flask equipped with stirrer, thermostated heating means, and a water cooled condenser for returning evaporated water. After 4 hours' treatment at 100° C. a sample of clear liquid was taken out and analysed as explained above. Within the expected accuracy of analysis, 100% of the initial amount of NaHPO₄ and 6.5% of Na₂ SO₄ were recovered in the resulting solution.

EXAMPLE 5 Adipate buffer

    ______________________________________                                         Initial mixture  NaHAd:       25 g                                                              Na.sub.2 SO.sub.4 :                                                                        100 g                                                              H.sub.2 O:   50 g                                             ______________________________________                                    

The experiment was carried out as described for the citrate buffer. The resulting solution was found to contain 100% NaHAd and 8.2% of Na₂ SO₄ in the initial mixture.

EXAMPLE 6 Phosphate buffer

    ______________________________________                                         Initial mixture  Na.sub.2 HPO.sub.4 :                                                                        50 g                                                              Na.sub.2 SO.sub.4 :                                                                        100 g                                                              H.sub.2 O:   90 g                                             ______________________________________                                    

The experiment was carried out as described for the citrate buffer. Within the accuracy of analysis the resulting solution was found to contain 100% of Na₂ HPO₄ and 14% of Na₂ SO₄ in the initial mixture.

The results in examples 4-6 illustrate that in this embodiment all the buffer will be recovered, while only Na₂ SO₄ will be removed from the system. The fact that some Na₂ SO₄ will accompany the buffer is insignificant, since there is no need for or desire to remove Na₂ SO₄ completely, but only to keep the Na₂ SO₄ concentration in the buffer solution at a suitably low level.

References

(1) NATO-CCMS STUDY. PHASE 1.1. Status Report on the Sodium Sulfite Scrubbing Flue Gas Desulfurization Process. US EPA, Washington DC. PEDCO Environmental, Cincinnati, Ohio. Contract No 68-01-4147, August 1978.

(2) DE-OS 25 06 666

(3) Pedroso, R. I. and Press, K. M. Sulphur recovered from flue gas at large coal fired power plants. In: "The Control of Sulphur and Other Gaseous Emissions", p F1-F20. 3rd Int. Symp., Salford (UK) 1979. EFCE Publ. Series No. 4.

(4) Keeth, R. S., Ireland, P. A. and Moser, R. E. Economic Evaluation of 24 FGD-Systems. EPA/EPRI FGD Symposium. Atlanta, Ga., Nov. 18-21, 1986.

(5) Ullmann 4. Auflage, Band 17, p. 211. 

I claim:
 1. A process for removing sodium sulphate from an aqueous citrate, adipate or phosphate buffer solution used for absorption of SO₂ from exhaust gases, which comprises evaporating the aqueous buffer solution at a temperature close to its normal boiling point to precipitate sodium sulphate, and separating the precipitated sodium sulphate from the buffer solution, and optionally subjecting the separated sodium sulphate to a treatment with water at elevated temperature to dissolve and recover accompanying buffer salts which are conveyed back to the buffer solution.
 2. The process according to claim 1, wherein a citrate buffer solution used for absorption of SO₂ is subjected to a stripping of SO₂ with steam, prior to the evaporation and separation.
 3. The process according to claim 1, wherein an adipate buffer solution used for absorption of SO₂ is subjected to a stripping of SO₂ with steam and subsequent cooling for precipitation of free adipic acid, prior to the evaporation and separation.
 4. The process according to claim 1, wherein a phosphate buffer solution is evaporated directly after the SO₂ absorption.
 5. The process according to claim 4, wherein the phosphate buffer solution is evaporated in two or more steps, a salt which primarily consists of sodium sulphate being precipitated in a first step, and a salt which in addition to sodium sulphate also contains phosphate buffer salts being precipitated in the subsequent step or steps, said salt precipitated in said subsequent step or steps being subjected to a treatment with water at elevated temperature to dissolve and recover phosphate buffer salts.
 6. The process according to any one of claims 1-5, wherein the buffer solution after SO₂ absorption is separated into a first stream containing a minor part of the buffer solution and a second stream containing a major part of the buffer solution, said first stream is subjected to said process for removing sodium sulfate, and the resultant purified buffer solution is mixed with said second stream so that the concentration of sodium sulphate in the resultant mixture is kept at no greater than 2.0M.
 7. The process according to claim 6, wherein the concentration of sodium sulphate in said resultant mixture is kept in the range 0.1-2.0M. 